Preheat the oven to 400 F, and position a rack in the center of the oven. Line a 12-cup muffin pan with paper liners, and lightly spray the liners with non-stick baking spray.
In a saucepan, combine the butter, milk and vanilla. Over medium heat, warm the liquid until the butter is melted and the milk is hot. Remove from the heat, stir in the raisins, and let sit for 10 minutes.
In a bowl, whisk together the flour, sugar, corn starch, baking soda, salt and spices.
Add the eggs, oil and the raisin/butter/milk mixture, and whisk until combined.
Divide the batter between the muffin cups, filling them nearly full.
When you put the muffins in the oven, immediately turn the temperature down to 375 F. Bake the muffins for about 16-18 minutes, until a toothpick or cake tester comes out clean.
Transfer the muffins to a cooling rack and cool completely before adding the frosting.
Fit a disposable piping bag with a coupler and a small star tip, such as #18.
Pipe a cross or X on top of the cooled muffins.
Enjoy the muffins on their own, or serve them with orange marmalade or apricot preserves.
Notes
Store leftover muffins in an airtight container at room temperature for up to 2 days.The muffins will be softest the day they're baked. You can soften the leftovers by reheating them for 20-30 seconds at 50% power in the microwave, but this will melt the buttercream.